THE NON-INSTRUMENT RATED PLT TOOK OFF ON A X-COUNTRY FLT AT NIGHT. WHEN ADVISED OF WX ALONG HIS ROUTE, HE REQUESTED & RECIEVED RADAR ADVISORIES. A SHORT TIME LATER, HE REPORTED THAT HE WAS IN A STORM. FOR ABOUT 10 MINUTES, THE RADAR CONTROLLER WAS COMMUNICATING WITH THE PLT WHILE PROVIDING ASSISTANCE. DURING THAT TIME, THE PLT REPORTED THAT HE WAS IN TERRIBLE WX CONDITIONS & HAD ENCOUNTERED BAD TURBULENCE. THE PASSENGER THEN BEGAN TO TALK ON THE RADIO FOR THE PLT. HE REPORTED THAT THEY WERE 'IN TROUBLE, IN BIG TROUBLE' & THAT THEY WERE TRYING TO FLY STRAIGHT & LEVEL. THE PASSENGER'S LAST TRANSMISSION WAS: 'WE'RE 2000 GOING UP & DOWN, WE'RE GOING DOWN.' A FARMER WAS AWAKEN BY THE ACFT CIRCLING HIS HOUSE. HE HEARD THE ACFT CLIMBING & DIVING BEFORE IT CRASHED. SUBSEQUENTLY, THE ACFT IMPACTED RISING GROUND IN AN AREA OF ROLLING TERRAIN. THE WITNESS SAID THE WIND WAS BLOWING & RAIN WAS FALLING. NO PREIMPACT MECHANICAL MALFUNCTION OR FAILURE OF THE ACFT WAS FOUND.